When Brendalee Flint got a call at work on Monday, Jan. 21, 2008, telling her that her 15-month-old daughter, Julieanna, was running a fever, she took the news in stride. But just to be safe, the mom of four later called her pediatrician, who assured her there was probably nothing to worry about. "That night, I gave her some medicine and put her to bed," she says.

But over the next couple of days, Julieanna's condition worsened: She would go from hysterically crying to suddenly appearing limp, and she had powerful bouts of diarrhea. So on Thursday, Flint drove her daughter from their Minnesota home to a hospital about 20 miles away. At first, doctors there suspected Julieanna had the flu, but by Friday, her pediatrician was sufficiently concerned that she scheduled more tests, including a CAT scan and a lumbar puncture, which showed that Julieanna had bacterial meningitis, a potentially deadly infection of the membranes covering the brain and spinal cord.

Flint was surprised by the diagnosis. "I'd never even thought of meningitis," she says. "It didn't seem like something I had to worry about." After all, since the availability of an infant vaccine in 1987, Hib meningitis cases in small children had nearly disappeared (before then, nearly 15,000 kids under age 5 contracted Hib meningitis every year, and more than 600 of them died). Brendalee had been vigilant about Julieanna receiving all her shots, including the one for Hib. What had gone wrong?

As it turns out, Julieanna's case was a perfect storm of events that illustrate a potential new health crisis in America. That year, Julieanna was one of five young children in Minnesota who contracted Hib, the state's first outbreak in more than 15 years, according to the CDC. The parents of three of the children had either skipped or delayed the Hib vaccine. The fourth was a 5-month-old too young to have been fully immunized; the fifth was Julieanna, who, unbeknownst to her parents, has a rare immune deficiency that made the Hib vaccine ineffective in her. Her doctors didn't immediately recognize the symptoms of the bacteria, perhaps because it had been so long since a case was reported in the area.

Unfortunately, that's starting to change. Over the past several years, the U.S. has seen outbreaks of not only Hib meningitis but also of other once prevalent childhood scourges like measles. A major cause, say experts, is the growing clusters of parents who are delaying or refusing standard vaccinations. But when parents choose not to vaccinate, they're not simply putting their own kids at risk; they're also unwittingly jeopardizing newborns, pregnant women, the elderly, and people like Julieanna with preexisting conditions. Those populations are increasingly vulnerable as diseases that we thought had been contained start to return, in larger numbers each year.

"There's a lot of mistrust out there"

From Edward Jenner's realization that cowpox could protect human beings from smallpox in 1796 to the introduction in the U.S. of the polio vaccine in 1955 and the measles vaccine in 1963, immunizations have allowed us to nearly — and, in the case of smallpox, completely — eliminate diseases that once killed thousands or even millions of people. As recently as the 1950s and 1960s, it was not uncommon for kids to be born disabled or placed in iron lungs as a result of childhood infections. How did lifesaving vaccines — once greeted with relief and open arms — turn into something so feared?

The current backlash dates to 1998, when British gastroenterologist Andrew Wakefield published a paper in the medical journal The Lancet hypothesizing a link between the MMR (measles, mumps, and rubella) vaccine and autism. His research, based on interviews with the parents of just 12 children, was widely criticized as speculative, but the press — and parents — seized hold.

A year later, a congressionally mandated inquiry highlighted the use of thimerosal, a mercury-based preservative, in some pediatric vaccines. There was no evidence it was harmful, but the Public Health Service and American Academy of Pediatrics (AAP), hoping to forestall parental anxiety, recommended its removal. (As a result, thimerosal has been absent from nearly all standard pediatric vaccines for a decade.) But the action backfired: A grassroots movement, led by about a dozen "mercury moms," began promoting their belief that many cases of autism were misdiagnosed cases of mercury poisoning.

Over the past decade, dozens of peer-reviewed studies that have collectively drawn on data from millions of children have consistently found no connection between vaccines and autism. In 2010 a multiyear investigation into Wakefield's work prompted The Lancet to fully retract his paper, and the U.K.'s General Medical Council revoked his British medical license.

Still, the damage was done, and rumors about vaccine safety have continued to spread, especially online, where anti-immunization sites argue that the shots can injure children. While one recent study showed that only about 2 percent of parents refuse all immunizations, more than 1 in 10 skip some vaccines or delay the age at which they're given. Increasingly, these parents tend to be clustered together, creating communities where vaccination rates may have dropped below the levels needed to keep infectious diseases at bay. Some of the lowest rates occur in affluent, well-educated communities like Boulder, Colo., and Marin County, Calif., where parents are often focused on being environmentally conscious and paying close attention to every aspect of their children's development.

"Sometimes, when you're surrounded by people who think the way you think, it's easy to believe that you're separated from the rest of the world," says George Wohlreich, M.D., CEO of the College of Physicians of Philadelphia. "If you never leave your house and no one ever comes to your house, you might be protected. But quite frankly, it's impossible to live that way."

Ironically, it is the very success of vaccines that may have led some parents to skip their children's shots."Today's parents didn't grow up seeing their peers affected by these diseases, or living in fear of being diagnosed themselves," says Amanda Dempsey, M.D., a pediatrician and researcher at the University of Colorado at Denver. "That can make immunizations start to feel more optional." But today many parents do know autistic children — making the perceived risks of vaccinating seem a lot more real than the diseases they prevent. "People who don't vaccinate do think they're doing the right thing for their children," Dempsey says. "There's a lot of mistrust because there's a lot of misinformation out there."

When Genevieve Futrelle's 7-month-old son was due for his first round of vaccines, "I just had this emotional reaction," says Futrelle, a first-time mom in New York City. "I didn't want him to get the [hepatitis B] shot. I was wary of how many shots kids get, and I just asked [the doctor], 'Do we need to do this?'" Futrelle decided to delay that injection, though she later consented to other standard vaccines. "I just didn't like the idea of all those chemicals going into his body. He's so fragile."

"It's a matter of safeguarding the community"

Experts say many parents may not realize that a vaccine's ability to stave off the spread of deadly diseases is most effective only when the vast majority are inoculated. This mechanism is called herd immunity: When enough people in a population are vaccinated against a disease — typically 85 to 95 percent — those unable to get vaccinated or whose immune systems are compromised are also protected. While parents who skip vaccines may benefit from herd immunity, they are also putting it in jeopardy.

"Getting vaccinated isn't only a matter of safeguarding yourself or your kids; it's a matter of safeguarding the community," says Anne Schuchat, M.D., director of the National Center for Immunization and Respiratory Diseases at the CDC. "Right now, as a country, we are so well protected compared to the rest of the world. But that can change." In France, she notes, the annual number of measles cases jumped from 40 or 50 to 15,000 in just a couple of years. "Enough unvaccinated people had accumulated, and suddenly the number exploded without a lot of warning," Schuchat says. "We don't want that to happen here. In 2011 we went from an average of 60 cases of measles a year to more than 200. The virus can come in from someone who has visited another country, and it's usually nipped in the bud, but only if everyone around that first patient is protected by vaccination."

One place where herd immunity can quickly be compromised is schools. Currently, at least 19 states allow parents to opt out of childhood inoculations for "philosophical" reasons (nearly every state allows a religious exemption, and all allow medical exemptions), and more and more families are doing so. In 2010 the San Diego Union-Tribune reported that the number of area parents who exempted their kindergartners from vaccines had quadrupled since 1990. In 2010, California schools experienced the highest rate of vaccine noncompliance in more than 30 years.

In April 2011, a small private school in Floyd County, Va., temporarily closed after 23 of its 45 students were infected with pertussis (whooping cough). "The outbreak was in an area where a lot of people weren't vaccinating," says Molly O'Dell, M.D., director of the area health district.

Multiple states have faced pertussis outbreaks this year, and reported cases are at their highest level in half a century. This is in part due to waning immunity in older children or adults who have not received boosters. In Washington State, a full-blown epidemic has already infected more than 4,000 people. (The state also has one of the highest vaccine exemption rates in the U.S., though new legislation aims to make it more difficult for parents to opt out.) Fortunately, no deaths have been reported so far in Washington — but in 2010 the disease killed 10 infants in California, all under 3 months old, too young to be fully vaccinated.

"Particularly with pertussis, the most vulnerable can't get immunized because they're under 1 month of age," says Schuchat. "Sometimes our most effective strategy is vaccinating around a person. For example, the elderly are very susceptible to influenza, and that's one reason we want people who work in hospitals and nursing homes to get the flu shot. Or in the case of measles, somebody brings the virus into the doctor's office and babies in the waiting room can become innocent victims."

"Our agenda is to protect kids"

"I try to stress to my patients that you can't predict when [vaccine-preventable diseases] are going to show up — and you can't protect your child any other way," says Ari Brown, M.D., an Austin, Tex., pediatrician and coauthor of the Baby 411 parenting book series. Brown, also a spokesperson for the AAP, says she's heard all sorts of rumors about vaccines, including that doctors get paid for each vaccine they administer. "Pediatricians have no hidden agenda; our agenda is to protect kids," she says. "We've been there with incredibly sick children, and when you see that, you want to do everything you can to stop it."

For an increasing number of pediatricians, that includes refusing patients whose parents won't adhere to the CDC's immunization schedule. A 2011 study found that 30 percent of Connecticut pediatricians had asked vaccine-refusing families to seek care elsewhere, as had 20 percent of Midwestern doctors in a recent survey (up from around 6 percent in 2001). "The biggest concern is that an unvaccinated child could expose other patients, especially newborns, to potentially deadly diseases," says Raymond Cattaneo, M.D., whose Kansas City, Mo., practice decided in 2008 to turn away families who wanted to skip or delay some immunizations. Another issue was "what felt like the contradiction of our supporting vaccines and saying we believed the science showing immunizations are safe and then letting parents make decisions based on fear," he says.